I've had a number of uneven temperature complaints this summer. Some of the problems involved loose wiring, bad damper motors or just poor ductwork installation.This is usual repair work, however the majority are from customers or clients who have only lived in their new homes for a few years The surprising thing is that these are newer homes and  the HVAC equipment is working perfectly. 

In my customer interview about how the systems are used, I've found that the zone damper system is not being used properly. No information about the HVAC zone system has been offered by whoever installed it. The customer is in the dark about how its supposed to work and so I'm left to explain why they can't keep one floor turned off and the other two at wildly different temperature settings.

I'm going to reprint one of the explanations I emailed to a client after a damper motor repair, which made it better but didn't solve it completely. The hope is that I won't have to apologize for another zone damper system which I don't advocate and did not install. If you know how it works you are better prepared to Save Your Money.

Mr. Client,

Zone damper systems as installed in Raleigh are problematic.The most common complaint is the one you just described.

One system big enough to cool the entire envelope of the structure is too big for any one zone by itself.

What the zone system was designed to do is compensate for temperature differences floor to floor.

 What it is misapplied to do is to give independent temperature control on each floor without respect for the whole envelope. Also without consideration for the fact that we have a one speed only unit. The equipment is designed to operate at its full capacity only, so various tricks are used to keep from starving the unit of air when only one zone calls for cooling.

It's like running a motor at high revolutions and controlling your speed with the clutch. It can work, but shifting to a different gear would be a better plan. This in equivalent would mean two or even three air conditioning units. It's easy to see why this isn't done by builders.

When codes caught up with the times in North Carolina, it was decided to use one unit per floor. This was cost prohibitive so a code buster was offered. Independent controls for each floor, all with one unit. 

The complaint  of hot upstairs and cold downstairs was addressed with controls rather than design.  You can tell I don't agree with this reverse engineering, but I'm a lowly furnace mechanic and do not get a vote.

The good news for contractors is that they do not have to make damper adjustments at the change of each season. Seasonal adjustment of dampers has been the only way to prevent stratification between floors.

The assumption was made that one temperature or small deviations around a common set point would be automatic. This is a workable solution for the hot upstairs problem if the floors are set fairly close together in temperature.

If you give people a control they will expect absolute control. If it can be adjusted we will stretch the ability of the control beyond its intention to the limit of its adjustment. If it shouldn't be set to 60 deg, then why does it go there? I don't have the answer, but it's a fair question.

The problem you described is real, and is exacerbated in direct proportion to the temperature set point difference between floors. The floors are not independent and there is no insulation between the separate areas anyway. The system works best when set points are close to each other and some spillage of air is expected between floors.

This may not be your case. I only describe a popular complaint. 

It shouldn't be your challenge to work around the systems design, and we aren't going to reinstall or reengineer the system so I think maybe I should try to tweak  the dampers resting positions around the way you use heating and cooling, insofar as possible of course.

I propose to do this very thing. If I invest a little of my time at no charge to you and get this thing working as close as possible to your requirement I will be a clever boy with a happy client. That's the big deal to me and is payment enough.

Results were that the homeowner didn't need me to come out again, once he understood what to expect he was comfortable with his system instead of worrying about some huge repair looming on the horizon.

At your service, Ed Cline